806 BELL SYSTEM TECHNICAL JOURNAL 



undue interference an adequate beacon service we can make no 

 assumption that any extra space can be found in the beacon band for 

 radio telephony. 



A radio telephone system with a sufficiently powerful transmitter 

 and sufficiently sensitive receiver to give reliable communication for 

 100 miles will give fair communication for perhaps 200 miles, and 

 its carrier wave will interfere with reception for a much greater dis- 

 tance. To avoid interference due to the beating of carrier frequencies, 

 airports within a few hundred miles of one another may be assigned to 

 different frequency channels, but serious difficulty is at once apparent 

 from a map of the National Airways. Within 800 miles of Chicago, 

 for example, there are over fifty terminal fields or airports. It would 

 seem obviously impractical to assign the available six telephone 

 channels to cover the eastern and central United States without 

 serious interference. By restricting power as much as possible and by 

 other means yet to be devised, it may be found possible to assign the 

 same wave-length to airports relatively nearer together. For the 

 distribution of weather information only, however, the airways may 

 well find insufficient the frequencies in the exclusive band, 315-350 

 kilocycles. 



On certain main routes, air transport companies will eventually 

 require two-way telephone despatching systems of their own to control 

 plane movements. These systems will consist of radio stations 

 situated at the various airports along the route and interconnected 

 by suitable wire lines. The frequency channels required for such 

 services cannot be found in the 315-350 kilocycles band which, as 

 just indicated, is apparently inadequate for the public services of 

 weather broadcasting from airports. Further channels in the short- 

 wave region appear to be necessary. 



In the short wave region Bell Telephone Laboratories have initiated 

 an additional development project. In cooperation with the Boeing 

 Air Transport Company, the Laboratories have undertaken to survey 

 the Chicago-San Francisco Airway and to develop a system of two- 

 way telephony between planes in flight and terminal landing fields 

 on this route. The Boeing Company planes and landing fields will 

 be equipped with experimental radio apparatus and a joint full-scale 

 experiment will be conducted during the winter of 1928-29. From 

 this work it is hoped to determine for an air transport company the 

 requirements for a two-way radio telephone service. The investi- 

 gation will furnish the basis for offering such facilities to other air 

 transport operators. 



This development of two-way radio-telephony on short waves is 



